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<h1 id="enum-representations"><a name="enum-representations" class="plugin-anchor" href="#enum-representations"><i class="fa fa-link" aria-hidden="true"></i></a>Enum representations</h1>
<p>Consider the following enum type:</p>
<pre><code class="lang-rust"><span class="hljs-meta">#[derive(Serialize, Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Message</span></span> {
Request { id: <span class="hljs-built_in">String</span>, method: <span class="hljs-built_in">String</span>, params: Params },
Response { id: <span class="hljs-built_in">String</span>, result: Value },
}
</code></pre>
<h2 id="externally-tagged"><a name="externally-tagged" class="plugin-anchor" href="#externally-tagged"><i class="fa fa-link" aria-hidden="true"></i></a>Externally tagged</h2>
<p>The default representation for this enum in Serde is called the externally
tagged enum representation. Written in JSON syntax it looks like:</p>
<pre><code class="lang-json">{<span class="hljs-string">"Request"</span>: {<span class="hljs-string">"id"</span>: <span class="hljs-string">"..."</span>, <span class="hljs-string">"method"</span>: <span class="hljs-string">"..."</span>, <span class="hljs-string">"params"</span>: {...}}}
</code></pre>
<p>The externally tagged representation is characterized by being able to know
which variant we are dealing with before beginning to parse the content of the
variant. This property allows it to work across a broad range of text and binary
formats. The <code>Serializer::serialize_*_variant</code> and
<code>Deserializer::deserialize_enum</code> methods use an externally tagged
representation.</p>
<p>This representation can handle any type of variant: struct variants like above,
tuple variants, newtype variants, and unit variants.</p>
<p>In JSON and other self-describing formats, the externally tagged representation
is often not ideal for readability. Serde provides attributes to select three
other possible representations.</p>
<p>All enum representations work in no-std projects, but externally tagged is the
only one that works in no-alloc projects.</p>
<h2 id="internally-tagged"><a name="internally-tagged" class="plugin-anchor" href="#internally-tagged"><i class="fa fa-link" aria-hidden="true"></i></a>Internally tagged</h2>
<pre><code class="lang-rust"><span class="hljs-meta">#[derive(Serialize, Deserialize)]</span>
<span class="hljs-meta">#[serde(tag = <span class="hljs-meta-string">"type"</span>)]</span>
<span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Message</span></span> {
Request { id: <span class="hljs-built_in">String</span>, method: <span class="hljs-built_in">String</span>, params: Params },
Response { id: <span class="hljs-built_in">String</span>, result: Value },
}
</code></pre>
<p>Written in JSON syntax, the internally tagged representation looks like this:</p>
<pre><code class="lang-json">{<span class="hljs-string">"type"</span>: <span class="hljs-string">"Request"</span>, <span class="hljs-string">"id"</span>: <span class="hljs-string">"..."</span>, <span class="hljs-string">"method"</span>: <span class="hljs-string">"..."</span>, <span class="hljs-string">"params"</span>: {...}}
</code></pre>
<p>The tag identifying which variant we are dealing with is now inside of the
content, next to any other fields of the variant. This representation is common
in Java libraries.</p>
<p>This representation works for struct variants, newtype variants containing
structs or maps, and unit variants but does not work for enums containing tuple
variants. Using a <code>#[serde(tag = "...")]</code> attribute on an enum containing a
tuple variant is an error at compile time.</p>
<p>The <code>serde</code> crate's "alloc" Cargo feature must be enabled to deserialize an
internally tagged enum. (It is enabled by default.)</p>
<h2 id="adjacently-tagged"><a name="adjacently-tagged" class="plugin-anchor" href="#adjacently-tagged"><i class="fa fa-link" aria-hidden="true"></i></a>Adjacently tagged</h2>
<pre><code class="lang-rust"><span class="hljs-meta">#[derive(Serialize, Deserialize)]</span>
<span class="hljs-meta">#[serde(tag = <span class="hljs-meta-string">"t"</span>, content = <span class="hljs-meta-string">"c"</span>)]</span>
<span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Block</span></span> {
Para(<span class="hljs-built_in">Vec</span><Inline>),
Str(<span class="hljs-built_in">String</span>),
}
</code></pre>
<p>This representation is common in the Haskell world. Written in JSON syntax:</p>
<pre><code class="lang-json">{<span class="hljs-string">"t"</span>: <span class="hljs-string">"Para"</span>, <span class="hljs-string">"c"</span>: [{...}, {...}]}
{<span class="hljs-string">"t"</span>: <span class="hljs-string">"Str"</span>, <span class="hljs-string">"c"</span>: <span class="hljs-string">"the string"</span>}
</code></pre>
<p>The tag and the content are adjacent to each other as two fields within the same
object.</p>
<p>The <code>serde</code> crate's "alloc" Cargo feature must be enabled to deserialize an
adjacently tagged enum. (It is enabled by default.)</p>
<h2 id="untagged"><a name="untagged" class="plugin-anchor" href="#untagged"><i class="fa fa-link" aria-hidden="true"></i></a>Untagged</h2>
<pre><code class="lang-rust"><span class="hljs-meta">#[derive(Serialize, Deserialize)]</span>
<span class="hljs-meta">#[serde(untagged)]</span>
<span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Message</span></span> {
Request { id: <span class="hljs-built_in">String</span>, method: <span class="hljs-built_in">String</span>, params: Params },
Response { id: <span class="hljs-built_in">String</span>, result: Value },
}
</code></pre>
<p>Written in JSON syntax, the untagged representation looks like this:</p>
<pre><code class="lang-json">{<span class="hljs-string">"id"</span>: <span class="hljs-string">"..."</span>, <span class="hljs-string">"method"</span>: <span class="hljs-string">"..."</span>, <span class="hljs-string">"params"</span>: {...}}
</code></pre>
<p>There is no explicit tag identifying which variant the data contains. Serde will
try to match the data against each variant in order and the first one that
deserializes successfully is the one returned.</p>
<p>This representation can handle enums containing any type of variant.</p>
<p>As another example of an untagged enum, this enum can be deserialized from
either an integer or an array of two strings:</p>
<pre><code class="lang-rust"><span class="hljs-meta">#[derive(Serialize, Deserialize)]</span>
<span class="hljs-meta">#[serde(untagged)]</span>
<span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Data</span></span> {
Integer(<span class="hljs-keyword">u64</span>),
Pair(<span class="hljs-built_in">String</span>, <span class="hljs-built_in">String</span>),
}
</code></pre>
<p>The <code>serde</code> crate's "alloc" Cargo feature must be enabled to deserialize an
untagged tagged enum. (It is enabled by default.)</p>
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